1. Field of the Invention
This invention relates generally to a foldable intraocular lens and more specifically to a foldable intraocular lens having at least one integrally formed haptic with a step edge configured to reduce posterior capsule opacification (PCO), stabilize the intraocular lens, and/or reduce the volume of the intraocular lens.
2. Description of the Related Art
Foldable ophthalmic lenses, such as intraocular lenses (IOLs), are used to replace the natural lens of an eye, for instance, when the natural lens is removed due to cataracts. Three-piece IOLs typically comprise an optic made of a soft, foldable material into which stiffer fixation members or haptics are staked to provide centering and stabilization of the lens. One-piece IOLs are also available in which the haptics or fixation members are integrally formed with the optic from the same foldable material as the optic.
One problem found with IOLs implanted in the capsular bag is that of posterior capsular opacification (PCO), a condition in which remnant equatorial lens epithelial cells (LECs) migrate between the posterior capsule surface and the posterior surface of the IOL. The incidence of PCO may be reduced by providing a continuous, sharp posterior corner around the entire posterior surface of the optic to impede cell migration into the lens area (e.g., U.S. Pat. Nos. 6,162,249 and 6,468,306, which are herein incorporated by reference). However, in the case of one-piece IOLs, haptic materials are generally soft, resulting in relatively bulky haptics that are thought to compromise the blockage of cell growth into the lens area. The haptics are especially bulky proximal to the lens, where greater rigidity is used to support the haptics and to help prevent the distal portions of haptics from sticking to the optic after insertion into the eye. Therefore, despite the use of sharp, square optic edges, the bulky haptics used in one-piece IOLs can lead to an increased incidence of PCO, as compared to similar three-piece IOLs (Miho Sugita, et al., American Journal of Ophthalmology, February 2004, vol. 137, no. 2, pp 377-379).
One reason for the increased prevalence of PCO in one-piece IOLs is thought to be that the relatively bulky haptics present an obstacle in attachment of the anterior capsule with the posterior capsule. This “shrink-wrap” effect may be advantageously used to pull the posterior capsule surface tight against the sharp posterior corner of the IOL. Ideally, the best corner seal should occur when there are no haptics present at all. The bulky haptics of a one-piece IOL move further from this ideal than the smaller haptic generally used in three-piece IOLs, thus providing a possible explanation of the higher incidence of PCO for one-piece IOLs.
In certain designs, a step edge is also formed on the optic in the vicinity of the haptics, providing in a continuous, 360 degree circular edge seal that surrounds the center of the lens and the posterior capsule. While this 360 degree edge seal configuration is relatively straightforward to implement in three-piece IOLs, it is more difficult for one-piece IOLs, since construction of an effective step edge in the area of the haptics can compromise other IOL design parameters. For instance, if the optic edge is made relatively thin, in order to reduce overall IOL volume, the haptic may be too thin to provide a desired amount of stiffness if a step edge is incorporated. Conversely, if the optic edge is made relatively thick, in order to provide a suitable attachment area for a thicker, stiffer haptic and to allow room for a suitable step edge, then the overall IOL volume may be too large, potentially leading to undesirably large incisions in the eye.
One way of helping to decrease the total IOL volume is disclosed in U.S. Pat. No. 5,476,513 to Brady et al., which is herein incorporated by reference. The '513 patent teaches an IOL having an optical zone surrounded by a peripheral zone, wherein the peripheral zone is thicker than the adjacent periphery of optical zone. Using this construction, the inlaid optical zone may be made thinner than it might otherwise be in an equivalent IOL not having a thicker peripheral zone. One potential problem with such an inlaid optical zone is that relatively sharp corners can contribute to undesired optical effects such as halos or visual poor contrast (e.g., FIG. 3A of the '513 patent). Such a design approach also does not address the problem of PCO.
Therefore, one-piece IOLs are needed that are simultaneously resistant to PCO, have an overall volume that is sufficiently small for delivery through a relatively small ocular incision, and produce a low level of scattered light.